Over the years, numerous methods have been proposed for delivering therapeutic agents into body and improving bioavailability of those medicinal agents. One of the attempts is to include such medicinal agents as part of a soluble transport system. Such transport systems can include permanent conjugate-based systems or prodrugs. In particular, polymeric transport systems can improve the solubility and stability of medicinal agents. For example, the conjugation of water-soluble polyalkylene oxides with therapeutic moieties such as proteins and polypeptides is known. See, for example, U.S. Pat. No. 4,179,337 (the '337 patent), the disclosure of which is incorporated herein by reference. The '337 patent discloses that physiologically active polypeptides modified with PEG circulate for extended periods in vivo, and have reduced immunogenicity and antigenicity.
Additional improvements have been also realized. For example, polymer-based drug delivery platform systems containing benzyl elimination systems, trimethyl lock systems, etc. were disclosed by Enzon Pharmaceuticals and Zhao, et al. as a means of releasably delivering proteins, peptides and small molecules. See also Greenwald, et al., J. Med. Chem. Vol. 42, No. 18, 3657-3667; Greenwald, et al., J. Med. Chem. Vol. 47, No. 3, 726-734; Greenwald, et al., J. Med. Chem. Vol. 43, No. 3, 475-487. The contents of each of the foregoing are hereby incorporated herein by reference.
More recently, PEG has been proposed for conjugation with oligonucleotides, especially oligonucleotides that are complementary to a specific target messenger RNA (mRNA) sequence. Generally, nucleic acid sequences complementary to the products of gene transcription (e.g., mRNA) are designated “antisense”, and nucleic acid sequences having the same sequence as the transcript or being produced as the transcript are designated “sense”. See, e.g., Crooke, 1992, Annu. Rev. Pharmacol. Toxicol., 32: 329-376. An antisense oligonucleotide can be selected to hybridize to all or part of a gene, in such a way as to modulate expression of the gate. Transcription factors interact with double-stranded DNA during regulation of transcription.
Oligonucleotides have also found use in among others, diagnostic tests, research reagents e.g. primers in PCR technology and other laboratory procedures. Oligonucleotides can be custom synthesized to contain properties that are tailored to fit a desired use. Thus numerous chemical modifications have been introduced into oligomeric compounds to increase their usefulness in diagnostics, as research reagents and as therapeutic entities.
Although oligonucleotides, especially antisense oligonucleotides show promise as therapeutic agents, they are very susceptible to nucleases and can be rapidly degraded before and after they enter the target cells making unmodified antisense oligonucleotides unsuitable for use in in vivo systems. Because the enzymes responsible for the degradation are found in most tissues, modifications to the oligonucleotides have been made in an attempt to stable the compounds and remedy this problem. The most widely tested modifications have been made to the back bone portion of the oligonucleotide compounds. See generally Uhlmann and Peymann, 1990, Chemical Reviews 90, at pages 545-561 and references cited therein. Among the many different back bones made, only phosphorothioate showed significant antisense activity. See for example, Padmapriya and Agrawal, 1993, Bioorg. & Med. Chem. Lett. 3, 761. While the introduction of sulfur atoms to the back bone slows the enzyme degradation rate, it also increases toxicity at the same time. Another disadvantage of adding sulfur atoms is that it changes the back bone from achiral to chiral and results in 2n diastereomers. This may cause further side effects. Still more disadvantages of present antisense oligonucleotides are that they may carry a negative charge on the phosphate group which inhibits its ability to pass through the mainly lipophilic cell membrane. The longer the compound remains outside the cell, the more degraded it becomes resulting in less active compound arriving at the target. A further disadvantage of present antisense compounds is that oligonucleotides tend to form secondary and high-order solution structures. Once these structures are formed, they become targets of various enzymes, proteins, RNA, and DNA for binding. This results in nonspecific side effects and reduced amounts of active compound binding to mRNA. Other attempts to improve oligonucleotide therapy have included adding a linking moiety and polyethylene glycol. See for example, Kawaguchi, et al., Stability, Specific Binding Activity, and Plasma Concentration in Mice of an Oligodeoxynucleotide Modified at 5′-Terminal with Poly(ethylene glycol), Biol. Pharm. Bull., 18(3) 474-476 (1995), and U.S. Pat. No. 4,904,582. In both of these examples, the modifications involve the use of linking moieties that are permanent in nature in an effort to stabilize the oligonucleotide against degradation and increase cell permeability. However, both of these efforts fail to provide any in vivo efficacy.
To conjugate therapeutic agents such as small molecules and oligonucleotides to polyalkylene oxides, the hydroxyl end-groups of the polymer must first be converted into reactive function groups. This process is frequently referred to as “activation” and the product is called an “activated polyalkylene oxide”. Other polymers are similarly activated.
In spite of the attempts and advances, further improvements in PEG and polymer conjugation technology such as activated polymers have therefore been sought. The present invention addresses this need and others.